An apparatus or gun for providing a controlled increase in muzzle velocity of a projectile while maintaining a safe maximum gas pressure inside a gun barrel has been developed during the past few years. The apparatus is a hybrid unit combining the technologies of liquid propellant with electrothermal technologies that avoids the disadvantages of these technologies when employed separately.
An elongated barrel is used in traditional guns, having a central bore closed at a breech end. A projectile is moved through the bore by heated gases produced by a burning propellant fired by an igniter. The burning propellant produces a relatively high pressure against the projectile when it is initially ignited, but the pressure decreases as the projectile moves along the gun barrel bore. Liquid fuel can be used to provide a more even pressure as the projectile moves, but it requires a critical fuel chamber size, bore diameter and manner of ignition of the fuel.
In liquid bipropellant technology one or more fluids are combined to generate a chemical reaction that produces pressure to power a projectile. The metering and mixing of the two fluids is difficult to control and therefore is subject to the risk of catastrophic failure or at least is subject to erratic performance. Mechanical means usually require seal and metering technology which is unreliable and so expensive as to be unjustifiable in a high production environment.
Electrothermal propulsion is a new technology that utilizes the electrical output of an inductive or capacitive network which condenses a pulse from an electrical generating source and energizes the system. Dielectric breakdown plasma is directed to a chamber containing an inert working fluid which vaporizes to provide gas pressure to eject or propel a projectile. All of the projectile energy is derived from the electrical power pulse. The resulting device is extremely bulky due to the excessive size of the electrical power supply which makes the unit difficult to integrate with projectile launchers.
The electrothermal-chemical (ETC) gun, which is employed with the propellants of the present invention, is in principle capable of providing significantly enhanced performance in comparison to guns utilizing chemical propellants alone, because the combination of electrical and chemical energy can provide a greater overall energy density and because the method by which the electrical energy is applied can be tailored to optimize the burning of the chemical propellant. The ETC concept is believed to provide the potential of increased muzzle kinetic energy (increased velocity at launch) within the constraints of the geometric configurations of current guns.
A genetic ETC gun works as follows: There is the discharge of a large electrical current from a power source into a plasma capillary, where a fuse wire is vaporized to create a high temperature (10,000.degree.-20,000.degree. K.) gas plasma. The vaporized plasma provides a narrow jet of ionized gas which vaporizes and entrains a portion of the fuel and causes the fuel to combine with a portion of an oxidizer material. The power supply continues to supply energy which controls the rate of vaporization of the plasma base and thus controls the rate of combustion of the oxidizer material and the fuel. Portions of the oxidizer material and fuel are launched and travel behind the projectile. Combustion of the travelling liquid phase occurs behind the projectile during the time it takes the projectile to move through the gun barrel. The combustion energy released by the travelling liquid causes pressure against the projectile to remain relatively constant as the projectile moves along the length of the gun barrel. This allows the breech and chamber pressures to be relatively low and still provide a high velocity projectile at the gun muzzle. As the electrical current continues to flow, the plasma temperature is maintained by ohmic heating. Wall material (such as polyethylene) is ablated because of the high temperatures. The pressure gradient between plasma capillary and combustion chamber forces the plasma to flow into the combustion chamber where it reacts with a propellant and generates hot gas, which is the working fluid that accelerates the projectile. The major purpose of the use of the electrical energy is to control the gas generation rate and the subsequent pressure history in the gun.
An electrothermal-chemical gun of the type that can be employed in the practice of the present invention is described in U.S. Pat. Nos. 4,711,154 and 4,895,062, that are incorporated herein in their entirety. In these patents the gun is referred to as a combustion augmented plasma (CAP) device that uses a plasma cartridge to controllably inject fuel into an oxidizer chamber. The plasma cartridge functions as an electric feed pump whose injection rate is controlled by the power applied to the plasma cartridge. The chemical reaction of the oxidizer with fuel supplied by the plasma feed pump provides the principal source of energy for generation or amplification of pressure. The uses of such generated pressure include the production of an impact force or the generation of a controlled pressure increase for use in propelling a projectile.
Chemical propellants having a high energy density are generally more useful in ETC gun systems since such propellants require correspondingly less electrical energy. Liquid propellants containing energetic ingredients are especially advantageous in comparison to the granulated solid propellants used in conventional guns, because a liquid can be loaded into a gun chamber with essentially no void volume, thus providing a higher energy density.
Previous research into liquid gun propellants that are ignited by conventional primers has proven unsuccessful because of uncontrollable and erratic behavior of the liquid. When no plasma discharge is present, the burning behavior of the liquid depends entirely on the hydrodynamic generation of surface area, which tends to be subject to random fluctuations. However, control of the plasma through design of the electrical pulse-forming network can largely overcome the effects of hydrodynamic fluctuations.
The more energetic liquid propellants found in the state of the art prior to the present invention, including the above references, consist of combinations of fuels and oxidizers. The oxidizers comprise either hydroxylammonium nitrate (HAN) or hydrogen peroxide. These oxidizers have the disadvantage that small amounts of certain impurities can catalyze their decomposition. Consequently, if propellants containing these oxidizers become contaminated in the course of handling or long term storage, their performance can be seriously compromised. In addition, HAN evolves small quantities of nitrogen oxides on storage, which can react with various organic compounds to adversely affect stability.
It is desirable for ETC propellants to have a high energy density and good long term stability under practical conditions of handling and storage. At the same time, ballistic performance must be acceptable. That is, the combination of chemical and electrical energy must be sufficient to provide the required projectile velocity and kinetic energy, while keeping pressure below a level that may damage the gun. A desirable kinetic energy level with a 30 mm gun is about 200 kilojoules. The maximum desired pressure depends upon the type of projectile being used. The maximum pressure with a noninstrumented projectile is about 500 MPa, while the maximum pressure with an instrumented or "smart" projectile is about 220 MPa. Control of the maximum pressure can be achieved by designing the propellant and electrical systems so as to limit the rate of pressure increase due to propellant burning. ETC propellants must be able to interact with electrical discharges to allow a relatively high level of pressure to be sustained in the gun as the projectile accelerates. This effect would permit the optimal level of ballistic performance to be achieved from a given size of gun. It is essential to tailor the physical and chemical properties of the propellant to provide good burning characteristics with electrical discharges produced under practical conditions. Propellants for the ETC gun application must also have consistent or controllable performance over a wide range of ambient temperatures. The propellant should be capable of being used at a low temperature, preferably as low as -40.degree. C. The propellant must also be sufficiently resistant to thermal decomposition so that it can be used and stored at high temperatures, preferably as high as 60.degree. C. In addition, it must not be capable of detonation under conditions of gun firing.